This invention relates to gas insulated transmission lines in which an inner conductor is provided concentrically within an outer sheath with a gaseous insulation medium therebetween and particularly to arrangements in which solid insulators support the inner conductor in relation to the outer sheath.
It is a practice for the inner conductor to have support insulators at axially spaced locations. A typical support insulator is cast circumferentially around the inner conductor, or onto a metal sleeve into which the conductor is inserted, and has three uniformly spaced posts or legs, referred to as a tri-post arrangement, extending toward the outer sheath. One insulator, usually at an end of the transmission line, is rigidly attached to the outer sheath. Others of the insulators are preferably not rigidly attached to the outer sheath in order to permit relative axial movement between the inner conductor and the outer sheath due to thermal effects.
The hardware for a movable insulator and the hardware for a fixed insulator have each previously required distinct metal elements cast intimately with the insulator. The movable insulator is characterized by having a metal casting disposed within a recess at the extremity of an insulator leg. The metal casting has a cup-shaped configuration and it further provides means for holding hardware including a roller element that allows the movable insulator and conductor to be rolled into place and to move in service. Reference is made to Hopkins U.S. Pat. No. Re. 31,949, July 16, 1985, for examples of such cast-in inserts and their assemblies. On the other hand, the fixed insulator has been characterized by having a casting that has a substantially flat surfaced insert without a recess in the manner of the movable insulator but with a tapped hole for securing to it a strap which then extends to the outer sheath and is joined such as by welding.
Rollers of a non-metallic material have been employed in accordance with the above-mentioned patent. The rollers were of nylon material, for example, which helps to minimize the risk of loose particles being produced by their movement that could lead to an electrical breakdown of the insulation medium. In the past the rollers were machined to a surface configuration with a curvature substantially matching that of the outer sheath. This has required variations in the rollers to match each of the differently sized outer sheaths.
The present invention has as one of its objects to simplify the construction of compressed gas insulated transmission lines by minimizing the number of different hardware elements required. It is desired both to reduce the numbers of kinds of parts required on a single transmission line and also to reduce the numbers of kinds of parts required for various size transmission lines.
An improvement is achieved by modifying the cast-in insert of the stationary insulator so it is a cup-shaped element which is the same as for the movable insulator.
The insert is configured with a recess so as to permit its use with a roller for the movable insulator. For the fixed insulator a metallic strap interconnector is secured by a fastener to the insert. The thicker, base portion of the strap is in direct engagement with the insert and an extended strap portion extends away from the insert. The extremity of the extended strap portion is joined, such as by welding, to the outer sheath. A button of a suitable non-metallic material, such as nylon, is pressed into a hole within the strap base portion to prevent particle generation during assembly of the insulator into the outer sheath during which time the assembly rides on the button which also provides a cushion and wear point during operation.
A further improvement is achieved by making rollers for the movable insulators that initially, as made and installed, have a simple cylindrical configuration and are not machined to conform to the curvature of the outer sheath. Such rollers may be used with various sized outer sheaths. In use, they exhibit a degree of plastic deformation which is itself, without machining, enough shaping to provide adequate support and roller functioning.
The design allows easy assembly and disassembly for repair or revision and minimizes the inventory required for components and spare parts.